Adjustable fluid-filled lens assembly and method for assembling the same

ABSTRACT

A method of assembling an adjustable fluid-filled lens assembly comprising biaxially tensioning an elastomeric membrane to a surface tension of greater than 180 N/m, typically greater than 1000 N/m; thermally conditioning the tensioned membrane, e.g., for one hour at a temperature of about 80° C., to accelerate relaxation of the membrane; mounting the membrane to a peripheral support structure whilst maintaining the tension in the membrane; assembling the mounted membrane with one or more other components to form an enclosure with the membrane forming one wall of the enclosure; and thereafter filling the enclosure with a fluid. The membrane may be formed from an aromatic polyurethane, and the fluid may be a phenylated siloxane. In some embodiments, the membrane is able to hold a substantially constant surface tension of at least 180 N/m for a period of at least 12 months.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/763,393, filed on Mar. 26, 2018, which is a national stageapplication, filed under 35 U.S.C. § 371, of International ApplicationNo. PCT/GB2016/000173, filed on Sep. 28, 2016, which claims the benefitof, and priority to, U.K. Application No. 1517160.6, filed Sep. 28,2015. The contents of each of these applications are expresslyincorporated herein by reference in their entireties.

The present invention relates to an adjustable fluid-filled lensassembly comprising a stretched elastomeric membrane, one face of themembrane forming a lens surface and the reverse face being disposedcontiguously in contact with a body of incompressible fluid forcontrolling the shape of the membrane, and has particular reference to amethod of assembling such an adjustable fluid-filled lens assembly inwhich the membrane is capable of holding tension for an extended periodof use. In another aspect, the present invention provides a method offorming a hard coating on one face of the membrane of such an adjustablefluid-filled lens assembly, and has particular reference to anadjustable fluid-filled lens assembly comprising a pre-tensionedelastomeric membrane having a coating on at least one face thereof whichis in compression to alleviate at least partially the force applied bythe tensioned membrane on a peripheral supporting structure such as oneor more bendable rings that hold the membrane around its edge.

Adjustable fluid-filled lens assemblies are known from WO 96/38744 A1,WO 98/11458 A1, WO 99/47948 A1, WO 01/75510 A1, WO 02/063353 A2, WO2006/055366 A1, WO 2007/049058 2, WO 2008/007077 A1, WO 2008/0501 14 A1,WO 2009/125184 A2, WO 2013/144533 A1, WO 2013/144592 A1, WO 2015/044260A1, U.S. Pat. Nos. 5,371,629 A and 6,040,947 A. According to each ofthese disclosures, a transparent, elastic membrane is held under tensionin contact with a body of fluid for controlling the shape of themembrane. Generally, the fluid is contained within a sealed enclosure,and the membrane forms one wall of the enclosure. The membrane has aninner face that contacts the fluid contiguously and an outer face thatforms an optical surface of the lens, with the optical power of the lensbeing related to the curvature of the membrane.

In one type of adjustable fluid-filled lens (“fluid injection”), asdisclosed in WO 91/17463 A1, WO 96/38744 A1, WO 98/11458 A1, WO 99/47948A1, WO 01/75510 A1, WO 02/063353 A2, WO 2007/049058 2, WO 2008/007077A1, WO 2008/050114 A1 or WO 2009/1251 84 A2, an adjusting amount fluidis selectively injected into or withdrawn from the enclosure to causethe membrane to distend outwardly or contract inwardly of the enclosurefor adjusting the curvature of the membrane. In another type ofadjustable fluid-filled lens (“fluid compression”), as disclosed in WO91/17463 A1, WO 2006/055366 A1, WO 2013/144533 A1, WO 2013/144592 A1, WO2015/044260 A1, U.S. Pat. No. 5,371,629 A or U.S. Pat. No. 6,040,947 A,the volume of fluid remains constant, but the enclosure is compressible,so that the distribution of the fluid within the enclosure can beadjusted by compressing or expanding the enclosure to cause the elasticmembrane to distend outwardly or contract inwardly.

It is known in the art to coat lenses with a variety of different typesof functional coating, including anti-scratch, anti-UV, anti-reflectiveand tinted coatings. Fluorinated polymer materials such, for example, asOF 210 (Canon Optron, Inc.), can be applied by vapour deposition to formhydrophobic and/or oleophobic coatings.

Adjustable fluid-filled lens assemblies may be used in eyeglasses toallow the optical power of one or both lenses to be adjusted. In someeyeglasses with adjustable lenses, a selectively operable controlmechanism associated with one or both lenses may be provided to allowthe wearer to adjust their optical power continually. The use of suchlenses in eyeglasses imposes a number of special requirements on thematerials that may be used for the membrane. In particular, in additionto being thin, elastic and transparent (at least across the visiblespectrum), the membrane material must also be colourless and be of lowtoxicity and low volatility; it should be inert, stable at hightemperatures and exhibit no phase changes within its normal range ofoperating temperatures. It should also exhibit low microbial growth.Further, the membrane material must be capable of forming an accurateand stable optical surface. Ideally, but not essentially, the membranematerial may also have a refractive index that is the same or similar tothat of the fluid. Suitably the fluid has a high refractive index(ideally at least about 1.45 or above 1.5, e.g., around 1.58±0.02) sothe lens is not unduly thick.

When used in eyeglasses, the elastic membrane is generally used in anupright orientation giving rise to a hydrostatic pressure gradientwithin the body of fluid; it may be subject to temperature variations ofup to about 50° C. and movement when the wearer moves. Desirably, themembrane should be pre-tensioned to a surface tension that is greatenough to reduce to an optically imperceptible level the variation inoptical power from top to bottom of the lens caused by the hydrostaticpressure gradient within the fluid and displacement of the fluid withinthe enclosure owing to inertia as the wearer moves. The membrane shouldbe capable of holding this tension stably to provide a substantiallyconstant load for an extended period of time, at least equal to theexpected life of the eyeglasses, which would normally be of the order ofyears, despite being subject to fluctuations in the surroundingtemperature and being held in constant contact with the fluid.

WO 2013/144592 A1 and WO 2015/044260 A1 disclose polyethyleneterephthalate (e.g. Mylar®), polyesters, silicone elastomers (e.g.polydimethylsiloxane), thermoplastic polyurethanes, includingcross-linked polyurethanes (e.g. Tuftane®), as suitable membranematerials for use in an adjustable lens assembly and silicone oils such,for example, as trimethylpentaphenyltrisiloxane andtetramethyltetraphenyltrisiloxane as suitable fluids. Thermoplasticpolyurethane in particular satisfies many of the special requirementsmentioned above, making it eminently suitable for use as a membrane inan adjustable lens. A problem with thermoplastic polyurethane however,is that silicone oils penetrate into the membrane material, causing themembrane to swell and lose tension.

It is an object of the present invention to provide an adjustablefluid-filled lens assembly of the kind described above, in which themembrane is capable of holding a constant surface tension that issufficient to reduce to an optically imperceptible level any variationin optical power across the lens resulting from an hydrostatic pressuregradient within the fluid and any displacement of the fluid within theenclosure owing to inertial effects for a period of at least 12 months.Preferably the membrane is capable of holding this surface tension, evenif the lens assembly is subjected to temperature fluctuations of 50° C.

In accordance with a first aspect of the present invention, there isprovided method of assembling an adjustable fluid-filled lens assemblycomprising biaxially tensioning an elastomeric membrane to a surfacetension of greater than 180 N/m; thermally conditioning the tensionedmembrane to accelerate relaxation of the membrane; mounting the membraneto a peripheral support structure whilst maintaining the tension in themembrane; assembling the mounted membrane with one or more othercomponents to form an enclosure with the membrane forming one wall ofthe enclosure; and thereafter filling the enclosure with a fluid.

The membrane may be biaxially tensioned to an initial surface tension ofat least 450 N/m or at least 500 N/m. In some embodiments, the membranemay be biaxially tensioned to an initial surface tension of at least1000 N/m. For example, the membrane may be biaxially tensioned to aninitial surface tension of about 1200 N/m.

In some embodiments, the membrane may be conditioned at a temperature ofat least 70° C. or at least 80° C. The membrane may be conditioned forat least 30 minutes, or at least 60 minutes. The thermal conditioningstep is suitably carried out before mounting the membrane to the supportstructure.

Thermal conditioning of the membrane may serve to accelerate relaxationof the membrane. After thermal conditioning, the membrane may have aresidual surface tension in the range about 180-550 N/m, depending onthe initial surface tension of the membrane, the properties of themembrane material and the specific conditions of the thermalconditioning step.

The membrane may be coated on at least one face with a barrier materialto form a barrier layer that may serve to prevent or retard the passageof the fluid. In some embodiments, the barrier material may be coated onan inner face of the membrane that contacts the fluid in the finishedassembly. In this arrangement, the barrier layer may serve to prevent orretard the passage of the fluid into the membrane.

Alternatively, the barrier material may be coated as a protective layeron an outer face of the membrane that is arranged outside the enclosurein the finished assembly, not in direct contact the fluid in theenclosure. In some embodiments, the outer face of the membrane may beexposed to the air. The membrane may be left uncoated with a barriermaterial on its inner face, so that the fluid may penetrate into themembrane material. By providing a protective layer on the outer face ofthe membrane, any fluid that penetrates into the membrane from theenclosure is prevented from leaking out of the membrane via its outerface, which would be undesirable because it might impair the opticalqualities of the lens, for instance by forming droplets on the outersurface.

The barrier material may be applied to the inner or outer face of themounted membrane as disclosed above after the thermal conditioning step.Conveniently, the material may be coated onto the mounted membranebefore it is assembled with the other components to form the enclosure.If desired, other coatings, such, for example, as an anti-reflectioncoating may be applied to the outer face of the membrane at this stage.Such other coatings may be single layer or multi-layer coatings, as isknown in the art.

The barrier material may comprise any suitable material for preventingor retarding the passage of fluid. The choice of barrier material maydepend on the particular fluid used. The refractive index of the barriermaterial is not important unless it is anti-reflective and/or thickenough to improve the surface quality of the membrane such, for example,as a self-levelling coating that is index-matched to the membrane. Itshould be capable of adhering well to the membrane and it should benon-yellowing. Desirably the barrier layer should be as thin aspossible. In some embodiments the barrier layer may have a thickness ofless than 20 nm, e.g. about 10 nm. In some embodiments, the barriermaterial may comprise a fluorinated polymer or a hydrophobic(oleophobic) polymer. Suitably said barrier material may selected fromethylene vinyl alcohol (EVOH), polyvinylidene chloride (PVDC), siliconeoxide (SiOx), polyacrylate, inorganic based coatings (e.g., MgF2) anddoped polymers (e.g., C-doped PTFE). Fluorine based polymeric homologuesof PTFE such, for example, as OF 210, which is commercially availablefrom Canon Optron, Inc., are preferred.

In a particular aspect of the present invention as described in moredetail below, the barrier material may comprise a functionalized polymersuch, for example, as an acrylate terminated polyurethane. In someembodiments, the barrier material may comprise a filler such asnanoparticulate silica. In some embodiments, the barrier material maycomprise an acrylic modified polyurethane silica hybrid coating.

The barrier material may be applied to the inner or outer face of themembrane by a variety of different techniques known in the art, but insome embodiments physical vapour deposition (PVD) may be used undervacuum. A coating of an acrylic-modified polyurethane barrier materialmay be applied to a face of the membrane by ultrasonic spraying toachieve a thickness in the range 0.5 μm-1.5 μm.

In embodiments in which the fluid is permitted to penetrate into themembrane material, for instance embodiments in which the inner face ofthe membrane is free of any barrier layer, the passage of fluid into themembrane material may cause the membrane progressively to swell andrelax, equivalent to a strain unloading of up to about 5%. The membranemay absorb up to about 20% of its own weight of fluid. In suchembodiments, the initial surface tension of the membrane may be selectedsuch that after thermal conditioning, the residual surface tension dropsto about 350-550 N/m. As the fluid penetrates into the membranematerial, the surface tension in the membrane may fall further. This isacceptable, provided the surface tension remains above about 180 N/m. Insome embodiments, the surface tension of the membrane may stabiliseafter the ingress of fluid into the membrane material at a final surfacetension in the range about 180-300 N/m, preferably 200-300 N/m.

In some embodiments, the finished assembly may be incubated at atemperature of at least about 40° C. to accelerate absorption of thefluid by the membrane. In some embodiments, the finished assembly may beincubated at a temperature of about 50-51° C. Suitably the finishedassembly may be incubated for a period of at least about 12 hours,preferably 24 hours.

Advantageously, it has been found that the membrane when biaxiallytensioned and thermally conditioned in accordance with the method of thepresent invention is able to hold a sufficiently constant tension of atleast about 180 N/m for a period of at least 12 months, typically atleast two years, even when disposed in continuous contact with the fluidand subjected to a variation in operating temperatures of about 50° C.By “sufficiently constant” herein is meant that the tension in themembrane varies by no more than about 25%, preferably no more than 20%,over the period.

According to a second aspect of the present invention, there is providedan adjustable fluid-filled lens assembly comprising a fluid-filledenclosure, one wall of which is formed by a tensioned elastomericmembrane that is mounted to a peripheral support structure; wherein themembrane is saturated with said fluid, is coated on its outer face witha barrier layer to said fluid, and the membrane holds a substantiallyconstant surface tension of at least 180 N/m.

As mentioned above the membrane may hold up to about 20% fluid by weightof the membrane.

According to a third aspect of the present invention, there is providedan adjustable fluid-filled lens assembly comprising a fluid-filledenclosure, one wall of which is formed by a tensioned elastomericmembrane that is mounted to a peripheral support structure; wherein themembrane is coated on its inner face with a barrier layer to said fluid,and the membrane holds a substantially constant surface tension of atleast 180 N/m.

Typically the membrane of the adjustable fluid-filled lens assembly ofthe third aspect of the invention is free of fluid.

In some embodiments, the membrane may hold a substantially constantsurface tension of at least 180 N/m for a period of at least 12 months.As mentioned above, by this is meant that the surface tension of themembrane does not vary by more than about 20% over this period.

Suitably, the membrane material should have a glass transitiontemperature below the usual operating range of the lens, preferablybelow about −5° C., and an elastic modulus in the range 10-200 MPa. Themembrane should be optically clear and non-toxic. In some embodiments,the membrane may have a refractive index of about 1.5. Various suitablepolymer materials are available to those skilled in the art, includingcross-linked urethanes and silicone elastomers, e.g., poly(dimethylsiloxane). Thermoplastic aromatic polyurethanes (TPUs) areparticularly preferred.

Thermoplastic polyurethanes are made up of block copolymer moleculesbisegmented hard and soft zones corresponding respectively tocrystalline and amorphous regions. It is this combination of flexible,amorphous segments with a high extensibility and low glass transitiontemperature, on the one hand, and rigid crystalline segments with a highmelting point, on the other hand, that gives the material itselastomeric nature. By altering the ratio of the crystalline phase, itis possible to vary properties such as hardness, strength, rigidity,extensibility and low-temperature flexibility over a broad range.Suitably, the membrane may be formed from a sheet of aromaticpolyurethane, which also has good microbe resistance. In someembodiments the polyurethane sheet may advantageously consist of apolyether or polyester aromatic polyurethane.

Thermoplastic polyurethanes can be produced by reacting (a) isocyanateswith (b) compounds that are reactive toward isocyanates and have amolecular weight of from 500 to 10,000 and, if appropriate, (c) chainextenders having a molecular weight of from 50 to 499, if appropriate inthe presence of (d) catalysts and/or (e) customary auxiliaries, asdisclosed in US 2008/0207846 A1, the contents of which are incorporatedherein by reference.

As organic isocyanates (a), it is possible to use generally knownaromatic, aliphatic, cycloaliphatic and/or araliphatic isocyanates,preferably diisocyanates; for example diphenylmethane 2,2′-, 2,4′-and/or 4,4′-diisocyanate (MDI).

As compounds (b) which are reactive toward isocyanates, it is possibleto use generally known compounds, such as diols and diamines, which arereactive toward isocyanates; for example polyetherols, which are usuallyreferred to as “polyols”, having molecular weights of from 500 to 12,000g/mol, preferably from 600 to 6,000 g/mol, in particular from 800 to4000 g/mol, and preferably a mean functionality of from 1.8 to 2.3,preferably from 1.9 to 2.2, in particular 2. A preferred polyol ispolytetramethyleneglycol (PTMG).

As chain extenders (c), it is possible to use generally known aliphatic,araliphatic, aromatic and/or cycloaliphatic compounds having a molecularweight of from 50 to 499, preferably 2-functional compounds; for examplealkanediols having from 2 to 10 carbon atoms in the alkylene radical, inparticular 1,4-butanediol.

To set the hardness of the TPUs, the molar ratios of the formativecomponents (b) and (c) can be varied within a relatively wide range.Molar ratios of component (b) to total chain extenders (c) to be used offrom 10:1 to 1:10, in particular from 1:1 to 1:4, have been found to beuseful, with the hardness of the TPUs increasing with increasing contentof (c).

Preferred TPUs are obtainable by reacting (a) isocyanates with (b)polyether diols having a melting point of less than about 150° C. and amolecular weight of from 501 to 8000 g/mol and, if appropriate, (c)diols having a molecular weight of from 62 g/mol to 500 g/mol.Particular preference is given to thermoplastic polyurethanes in whichthe molar ratio of the diols (c) having a molecular weight of from 62g/mol to 500 g/mol to the component (b) is less than 0.2, particularlypreferably from 0.1 to 0.01.

A particularly preferred polyether polyurethane for use in the membraneof the lens assemblies of the present invention is formed fromdiphenylmethane-4,4′-diisocyanate (MDI), polytetramethylene glycol and1,4-butanediol having a Shore A hardness of about 86, a density of about1.12 g/cm3, a tensile strength of about 33 MPa and a tear strength ofabout 105 N/mm. This material is commercially available from BASF underthe trade mark Elastollan® 1185.

Generally, the fluid should be substantially incompressible. It shouldbe transparent and colourless, with a refractive index of at least about1.5. Suitably the refractive index of the membrane and fluid should bematched, so that the interface between the membrane and fluid issubstantially imperceptible to the user. The fluid should have lowtoxicity and low volatility; it should be inert and exhibit no phasechange above about −10° C. or below about 100° C. The fluid should bestable at high temperatures and exhibit low microbial growth. In someembodiments, the fluid may have a density of about 1 g/cm3.

Various suitable fluids are available to those skilled in the art,including silicone oils and siloxanes such, for example, as phenylatedsiloxanes. A preferred fluid is pentaphenyltrimethyltrisiloxane.

In some embodiments, the membrane may suitably comprise a polyetherpolyurethane such, for example, as the above-mentioned materialavailable under the trade mark Elastollan® 1185, and the fluid maycomprise a silicone oil or phenylated siloxane, such aspentaphenyltrimethyltrisiloxane. The refractive indexes of the membranematerial and fluid are suitably the same or substantially the same andare at least 1.5.

In addition to the membrane, the enclosure may comprise a receptacle forreceiving the fluid. The receptacle may be closed by the membrane, whichforms one wall of the enclosure. The receptacle is suitably made from amaterial that is optically clear and colourless and has a refractiveindex of at least about 1.5. The refractive index of the receptacle issuitably matched to the refractive index of the membrane fluid, so thatthe boundary between the receptacle and the fluid is substantiallyimperceptible to the user. In some embodiments, the receptacle may berigid, for instance when the fluid-filled lens assembly of the presentinvention is of the “fluid injection” type. Alternatively, thereceptacle may be compressible, particularly when the lens assembly ofthe invention is of the “compression” type. In the latter case, thereceptacle may comprise a flexible peripheral wall that may buckle inthe manner of bellows to allow the receptacle to be compressed. Asuitable flexible material is a transparent, thermoplastic polyurethanesuch, for example, as Tuftane®.

In some embodiments, the peripheral support structure for the membranemay comprise one or more rings that are arranged to hold the membranearound its edge. The ring or rings may be substantially rigid or theymay be bendable. In some embodiments, the membrane may be non-circular,and the ring or rings for supporting the membrane may be bendable toallow displacement of the edge of the membrane out of plane when theassembly is actuated, to cause or allow the membrane to distend orcontract spherically or according to another Zernike polynomial of thekind typically used in optical or ophthalmic applications.

A problem that may arise in some embodiments that include one or morebendable rings as part of the peripheral support structure for themembrane is that the surface tension in the membrane may tend to causeunwanted in-plane bending of the one or more rings, as opposed to out ofplane bending which is required, as mentioned in the precedingparagraph, to allow displacement of the edge of the membrane out ofplane when the assembly is actuated to cause or allow the membrane todistend or contract spherically or in accordance with one or more otherZernike polynomials. WO 2013/143630 A1 discloses a deformable membraneassembly comprising one or more bending controllers acting on a bendablemembrane supporting member to control in-plane bending of the supportingmember in response to loading through tension in the membrane. Whilstthe bending controllers of WO 2013/143630 A1 are satisfactory, theyrequire additional components of the assembly, which adds to itscomplexity and cost of manufacture. They also take up significant volumeand weight within the assembly.

A different object of the present invention therefore is to provide animproved way of controlling the in-plane bending of the bendable ring orrings of the peripheral support structure, which is simpler tomanufacture and occupies less volume and/or weight in the finishedassembly.

In accordance with a fourth aspect of the present invention, therefore,there is provided an adjustable fluid-filled lens assembly comprising afluid-filled enclosure, one wall of which is formed by a tensionedelastomeric membrane that is mounted to a peripheral support structurecomprising one or more bendable rings arranged to hold the membranearound its edge; wherein the membrane is coated on at least one of itsinner and outer faces with a coating that is formed of a material havinga higher modulus than the membrane and is disposed under compression tocounteract the tension in the membrane and thereby at least partiallyalleviate the in-plane force applied by the membrane to the one or morerings. In some embodiments, the one or more rings may be non-circular.

As mentioned above, the membrane may suitably have an elastic modulus inthe range 10-200 MPa.

The membrane may have a thickness in the finished fluid-filled lensassembly in the range 100-300 μm. In some embodiments the membrane mayhave a thickness in the range 150-250 μm, preferably about 200-220 μm.As mentioned above, the membrane may hold tension in the finishedassembly in the range 180-300 N/m, preferably 200-300 N/m.

The coating may have an elastic modulus between one and two orders ofmagnitude greater than the elastic modulus of the membrane. Forinstance, the coating may have an elastic modulus of at least 0.1 GPa,suitably at least 0.5 GPa, and more suitably at least 0.75 GPa or 1 GPa.In some embodiments, the coating may have an elastic modulus of about 1GPa.

The thickness of the coating may be calculated to provide a substantialalleviation (reduction) in the tension that is applied to the bendablering or rings. In most embodiments, a coating having a thickness that iscalculated to counteract completely the surface tension in the membranewould be undesirably thick, but a coating having a thickness in therange 0.5-1.5 μm, for example 1 μm, may be sufficient still to have asignificant effect on the mechanics of the membrane. In some embodimentsthe coating may have a thickness in the range 1-1.5 μm, preferably1.2-1.5 μm.

Suitably, the coating may be formed from a material that is compatiblewith the membrane material to provide strong interfacial forces betweenthe face of the membrane and the coating. As mentioned above,thermoplastic aromatic polyurethanes (TPUs) are preferred materials forthe elastomeric membrane. TPUs are hydrophobic, and a problem with usinganother hydrophobic coating material such as a fluorinated polymerapplied by PVD, for example, is that there are no interfacial bonds and,as a result, the PVD coating is fragile.

In accordance with the present invention, the coating may also comprisea polyurethane material that is capable of forming strong interfacialbonds with the membrane. Advantageously, the polyurethane coatingmaterial may comprise cross-linkable acrylate groups to allow thecoating to be cured after application to the membrane. The coatingmaterial may comprise an acrylic-modified polyurethane such, forexample, as a polyether or polyester polyurethane acrylate. In someembodiments, the coating material may comprise an acrylic-modifiedaliphatic or aromatic polyurethane. A polyurethane coating may also beused with other membrane materials of the kinds mentioned above,including for example siloxane membranes.

Examples of suitable polyester urethane acrylates include productsformed by the reaction of an hydroxyl functional polyester acrylate withan isocyanate functional material. Polyester acrylates may includereaction products of polyester polyols with acrylic acid.

Suitable isocyanate functional components include hexamethylenediisocyanate, isophorone diisocyanate, isocyanate functional acrylicpolymers and polyurethanes, reaction products of hydroxyl functionalcomponents (e.g. poly-ethylene glycol, poly-propylene glycol and di-,tri- and higher hydroxy functionality aliphatic alcohols (e.g. glyceroland trimethylolpropane) and their ethoxylated, propoxylated andpolycaprolactone analogs) with di-, tri- and etc. isocyanates (e.g.hexamethylene diisocyanate, isophorone diisocyanate and toluenediisocyanate (TDI)).

Specific examples of suitable polyurethane acrylate coating materialsare RAYCRON® CeranoShield UV Clearcoat and G-NT200, which arecommercially available from PPG Industries, Inc., Barberton SpecialtyChemicals Plant of Barberton, Ohio, and Lens Technology International ofLa Mirada, Calif., respectively.

Advantageously, the coating material may comprise nanoparticulate silicato afford additional stiffness. The silica filler may also providescratch-resistance. Advantageously, the coating material may comprise50-60% wt. silica, e.g. about 52% wt., but in some embodiments a silicaconcentration of about 25% wt. may suffice. In some embodiments thecoating material may be diluted with an appropriate solvent to achieve athinner coating on the membrane, for instance in the range 0.4-0.5 μm.The choice of solvent will vary according to the selected coatingmaterial, but typically an acetate or alcohol may be used. In such casesthe concentration of silica may be reduced by dilution to 7-10% wt.

The polyurethane acrylate coating material may further comprise asuitable photoinitiator such, for example, as a free radicalphotoinitiator.

The polyurethane coating material may be applied to the face of themembrane by ultrasonic spraying, which has been found to achieve athickness well below 1 μm if desired. In ultrasonic spraying, a mass ofliquid is atomised to form tiny droplets which are then sprayed over asubstrate in the form of a thin film.

In accordance with a fifth aspect of the present invention thereforethere is provided a method of assembling an adjustable fluid-filled lensassembly comprising biaxially tensioning an elastomeric membrane;thermally conditioning the tensioned membrane to accelerate relaxationof the membrane; mounting the membrane to a peripheral support structurewhilst maintaining the tension in the membrane; coating a face of themembrane with a cross-linkable polyurethane acrylate coating material;curing the coating material; assembling the mounted membrane with one ormore other components to form an enclosure with the membrane forming onewall of the enclosure; and thereafter filling the enclosure with afluid.

The membrane may be formed from any suitable elastomeric material asdisclosed above, including cross-linked urethanes and siloxaneelastomers, e.g., poly (dimethylsiloxane). Thermoplastic aromaticpolyurethanes (TPUs) are particularly preferred.

As described above, the membrane may be tensioned to an initial surfacetension of about 1200 N/m. After thermal conditioning, the membrane mayhave a residual surface tension in the range about 180-550 N/m.

Advantageously, the coating material may comprise a nanoparticulatesilica filler as described above and, when cured, may have an elasticmodulus of at least 0.5 GPa. The coating may be applied to the face ofthe membrane to a thickness of about 0.5-1.5 μm. Suitably the coatingmay be applied to the outer face of the membrane to impart scratchresistance and cleanability to the assembly.

Advantageously, the face of the membrane may be activated prior toapplication of the coating material to reduce the contact angle of theface to allow better adhesion of the coating material. Suitably themembrane face may be activated by plasma treatment, for example airplasma. Thermoplastic polyurethane is hydrophobic in nature and has atypical contact angle in the range 95-105°. Activating a thermoplasticpolyurethane membrane face by plasma treatment reduces the contact angleto about 78-83°.

After applying the coating material to the face of the membrane, thecoating material may be cured. Suitably UV exposure may be employed forthis purpose. For example, curing may be effected using Mercury vapourH-Bulb which outputs UV light in the range 220-320 nm, with a spike inthe longwave range of 365 nm. Curing proceeds by activation of thephotoinitiator within the coating, which triggers cross-linking of theacrylate moieties within the polyurethane acrylate material, resultingin a hard coating on the face of the membrane.

After applying the coating, assembling the mounted membrane with one ormore other components to form the enclosure and filling the enclosurewith the fluid, the finished assembly may be incubated at a temperatureof at least about 40° C., as described above, allowing the membrane torelax slightly, thereby causing the coating to be compressed asdescribed above. Compression of the coating may act to resist furtherrelaxation of the membrane, thereby reducing the in-plane force appliedby the membrane to the peripheral supporting structure for the membrane.

Suitably the adjustable fluid-filled lens assembly of the invention maybe used in a pair of spectacles. Accordingly, the present inventionprovides, in a sixth aspect, a pair of spectacles comprising at leastone adjustable fluid-filled lens assembly in accordance with theinvention.

Following is a description by way of example only of embodiments of thepresent invention.

FIG. 1 is a schematic perspective view of an adjustable fluid-filledlens assembly according to the present invention.

FIG. 2 is a schematic perspective view of the adjustable fluid-filledlens assembly of FIG. 1 shown in cross-section.

FIG. 3 is a schematic exploded view showing in cross-section thecomponents of the lens assembly of FIG. 1 .

FIG. 4 is a schematic perspective view of a thin sheet of viscoelasticmaterial mounted on a circular clamp and a press for stretching thesheet, shown in cross-section.

FIGS. 5A-5L show a sequence of steps for assembling an adjustablefluid-filled lens assembly with a biaxially tensioned membrane inaccordance with the present invention.

FIGS. 6A, 6B and 6C are illustrative graphs based on empirical datashowing respectively the change in thickness (in μm), tension (in N/m)and stress (in MPa) of the membrane during the principal manufacturingsteps.

FIG. 7 is a scattergram of the measured line tensions with time forsixty-six individual polyurethane membranes that have been tensioned andthermally conditioned in accordance with the present invention and heldin continuous contact with a body of silicone oil.

FIGS. 1, 2 and 3 show schematically an adjustable fluid-filled lensassembly 10 of the kind known in the art. The lens assembly 10 of FIGS.1 to 3 is of the “compression type” referred to above, in that itcomprises a body of incompressible fluid 60 of fixed volume, and thefocal power of the lens is controlled by compression of the assembly 10in the manner described below to redistribute the fluid behind a thin,elastic membrane 12 membrane to cause the membrane to distend orcontract thereby changing its curvature. The present invention isequally applicable to adjustable fluid-filled lens assemblies of the“fluid injection type”, which also comprise a similar membrane.

Only the parts of the assembly that are directly relevant to the presentinvention are shown in the interests of brevity. Additional features,such for example as the control mechanism for selectively controllingthe refractive power of the assembly 10 are briefly mentioned below, butare omitted from the drawings.

As shown in FIGS. 2 and 3 , the membrane 12 has an outer front face 14and an inner rear face 16 and is stretched and mounted between a frontring 18 and a rear ring 20, which serve as a peripheral supportstructure for the membrane 12, holding the membrane 12 under tensionaround its edge as described in more detail below.

The membrane 12 comprises a sheet of a thermoplastic polyurethane. Inthe present embodiment, the membrane comprises a sheet cf a polyetherpolyurethane formed from diphenylmethane-4,4′-diisocyanate (MDI),polytetramethylene glycol and 1,4-butanediol having a Shore A hardnessof about 86, a density of about 1.12 g/cm³, a tensile strength of about33 MPa and a tear strength of about 105 N/mm. This material iscommercially available from BASF under the trade mark Elastollan® 1185A10. The sheet has an initial thickness of about 380 μm, but in thefinished assembly has a thickness of about 220 μm, as described in moredetail below. Other grades of thermoplastic polyurethanes may be used;for instance, a polyether polyurethane in which the relative proportions(stoichiometry) of the isocyanate, polyol and chain extender componentsare varied to afford a different a Shore hardness. Alternatively, themembrane may comprise a polyether polyurethane made from a differentisocyanate, polyol and/or chain extender. More generally, the membranemay be formed from any suitable thermoplastic polyurethane material or adifferent viscoelastic polymer material, provided it is optically clear,has a glass transition temperature below the usual operating range ofthe lens, typically below about −5° C., an elastic modulus in the range10-200 MPa, is inert and non-toxic, exhibits low microbial growth and iscapable of being bonded to the rings 18, 20.

In the present embodiment the outer face 14 of the membrane 12 is coatedwith a protective layer of a barrier material (not shown) for thepurpose described below. Any suitable hydrophobic coating material maybe used, e.g. a fluorinated polymer. The coating material should becapable of adhering well to the membrane 12. It should be non-yellowing,and the barrier layer should be as thin as possible. In someembodiments, the barrier layer may have a thickness of about 10 nm, butthose skilled in the art will appreciate that the thickness may bevaried according to the nature of the coating material used and thedesired properties of the lens assembly 10. In one embodiment, afluorine based polymeric homologue of PTFE that is commerciallyavailable from Canon Optron, Inc. under the trade mark OF 210 is used.

In another embodiment, the barrier material comprises a layer ofcross-linked polyurethane acrylate, which may optionally include ananoparticulate silica filler as described in more detail below. In thisother embodiment, the layer has a thickness of about 1 μm, but againthis may be varied according to the nature of the coating material andthe desired properties of the lens assembly 10. Thus, in alternativeembodiments, the barrier layer comprising a silica-filledacrylic-modified polyurethane may have a thickness in the range 0.5-1.5μm.

The membrane 12 is shaped and dimensioned as a lens, with the outersurface 14 of the membrane 12 serving as an optical surface of the lens.The membrane 12 can be any shape as desired. In some embodiments, thelens 10 may be used in a pair of spectacles, in which case the membrane12 will be suitably shaped and dimensioned for that application. Forinstance, the membrane 12 may be circular, or it may be generally ovalor rectangular. Numerous different lens shapes for spectacles are knownin the art. In the present embodiment, the membrane 12 is generallyrectangular, having rounded corners. Only about half of the assembly 10is shown in FIGS. 2 and 3 .

In embodiments, such as the present embodiment illustrated in FIGS. 1 to3 , in which the membrane is non-round, the rings 18, 20 should bebendable out of the plane of the membrane 12, as described in WO2013/144533, the contents of which are incorporated herein by reference,to cause or allow the membrane to distend or contract spherically inuse, or in accordance with another Zernike polynomial of the kindtypically prescribed for ophthalmic use. In the present embodiment, therings 18, 20 are fabricated from a sheet of stainless steel; the frontring 18 has a thickness of about 0.25 mm and the rear ring 20 athickness of about 0.15 mm. In embodiments in which the membrane 12 iscircular, the rings 18, 20 are not required to be bendable; the membranemay be held by a rigid peripheral support structure, which is moreconvenient for holding the membrane 12 under tension.

The membrane 12 is glued between the front and rear rings 18, 20.Suitable adhesives are known to those skilled in the art such, forexample, as light curable adhesives. In the present embodiment, Delo®MF643 UV curing epoxy adhesive is used.

The rear ring 20 is glued to a peripheral lip 22 of a dish-shapedreceptacle 24. The same adhesive may be used as for attaching the rings18, 20 to the membrane 12. The dish-shaped receptacle 24 comprises arear wall 26 having a shape that corresponds to the shape of themembrane 12 and a peripheral side wall 28 that extends forwardly of therear wall and terminates in said peripheral lip 22. The dish-shapedreceptacle is made of a flexible, transparent thermoplastic polyurethanesuch, for example, as Tuftane® (available from Messrs. PermaliGloucester Ltd, Gloucester, UK) and is about 50 μm thick; other similartransparent materials may be used, e.g., DuPont® boPET(biaxially-oriented polyethylene terephthalate) and the thicknessadjusted accordingly.

In some embodiments, the assembly 10 may comprise an annular supportdisc (not shown) of the kind described in WO 2013/143630, the contentsof which are incorporated herein by reference, that is interposedbetween the rear ring 20 and the lip 22 for reinforcing the rings 18, 20against unwanted “in-plane” buckling under the tension in the membrane12. In the other embodiment mentioned above, in which the membrane 12 iscoated with a layer of silica-filled cross-linked polyurethane acrylate,the annular supporting disc may be omitted.

The rear wall 26 of the dish-shaped receptacle 24 has a rear face 30(see FIG. 3 ) that is bonded to a planar front face 32 of a rear lens 34of fixed refractive power. The rear lens 34 is a meniscus lens having aconcave opposite rear face 36. The rear face of the dish-shapedreceptacle 24 is bonded contiguously to the front face 32 of the rearlens 34 by means of a transparent pressure-sensitive adhesive (PSA)such, for example, as 3M® 821 1 adhesive. In the present embodiment, alayer of PSA about 25 μm thickness is used, but this may be varied asrequired.

The dish-shaped receptacle 24, rear ring 20 and membrane 12 thus form asealed enclosure 54. The enclosure 54 is filled with an incompressiblefluid 60 through a fill-port (not shown) let into the side wall 28 ofthe dish-shaped receptacle 24. In the present embodiment, the fluid ispentaphenyltrimethyltrisiloxane, which is a phenylated siloxane, butother suitable silicone oils and other fluids are available to thoseskilled in the art. The fluid should be colourless, with a highrefractive index of at least 1.45 or 1.5. In the present embodiment, thefluid has a refractive index of about 1.58±0.02; it should have lowtoxicity and low volatility; it should be inert and exhibit no phasechange above about −10° C. or below about 100° C. The fluid should bestable at high temperatures and exhibit low microbial growth. Generallythe fluid has a density of about 1 g/cm3. As described in detail below,the enclosure 54 is filled with the fluid 60 under vacuum to ensure noair is present. Further, the enclosure 54 may be over-filled to distendthe membrane 12 slightly to ensure the enclosure 54 is filled completelywith the fluid 60, such that the fluid contacts whole of the inner face16 of the membrane 12 continuously, with no gaps between the membrane 12and the fluid 60.

The filled enclosure 54 is compressible owing to the flexibility of theside wall 28 of the dish-shaped receptacle 24 and the elasticity of themembrane 12. Compressing the enclosure against the rear lens 34 causesthe side wall 28 of the dish-shaped receptacle 24 to buckle, which inturn causes the membrane to distend outwardly to accommodate theincompressible fluid 50, thereby changing the curvature of the membraneas disclosed, for example, in WO 2013/144533.

The rear lens 34, dish-shaped receptacle 24, rings 18, 20 and membrane12 are accommodated within a housing 40 comprising a front retainer 48and a rear retainer 46 that are made of metal and glued together at 47to form an internal recess in which the rear lens 34, dish-shapedreceptacle 24, rings 18, 20 and membrane 12 are received. The rearretainer 46 has a circumferential side wall 43 having an inner surface44 that is formed with an intermediate step 42. The rear lens 34 isglued to the inner surface 44 towards a rear end of the rear retainer46, such that the front face 32 of the rear lens 34 is level with saidstep 42, where the inner surface 44 of the side wall 43 is steppedoutwardly to provide a clearance between the side wall 28 of thedish-shaped receptacle 24 and the inner surface 44 forwards of the step42 to accommodate the side wall 28 as it buckles in use, as well asparts of a control mechanism (omitted from the drawings for simplicity)for selectively compressing the filled enclosure 54 against the rearlens 34 in the manner described above.

The front retainer has a turned-in front rim 50 that is spaced forwardlyof the rings 18, 20 and membrane 12 to allow the membrane to distendforwardly in use.

Depending on the shape of the membrane 12, the rings 18, 20 may behinged to the housing 40 at one or more hinge points as disclosed in WO2013/144533 or WO 2013/144592, the contents of which are incorporatedherein by reference. The control mechanism may include one or moreactuators that are mounted to the housing 40 in engagement with therings 18, 20 (or parts attached to the rings) at predetermined controlpoints around the rings 18, 20 to move the rings towards or away fromthe rear lens 34 at the control points, as disclosed in WO 2013/144592or WO 2015/044260, the contents of which are incorporated herein byreference. In this way, the assembly may be selectively actuated tocause the membrane to distend outwardly or contract inwardly in relationto the enclosure to control the curvature of the outer face 14 of themembrane 12.

The assembly 10 thus forms a compound lens with a number of internal andexternal optical surfaces. The total refractive power of the assembly 10is determined by the curvature of the rear surface 36 of the fixed rearlens 24 and the curvature of the outer face 14 of the membrane 12.Preferably the materials for the membrane 12, the dish-shaped component24 and fluid 60 are selected to have as closely as possible the samerefractive index, so that the interfaces between the membrane 12 and thefluid 60, and between the fluid 60 and the rear wall 26 of thedish-shaped component are almost invisible to the eye when viewedthrough the assembly 10.

The membrane 12 is held under tension to stabilise it againstdeformation. An untensioned or inadequately tensioned membrane would besusceptible to external vibrations, to inertial effects when subjectedto acceleration or deceleration in use, and to external forces such asgravity. When used in a pair of eyeglasses, for example, the membrane 12is subject to continual movement and is worn in in a generally uprightorientation which gives rise to a hydrostatic pressure gradient in thefluid 60. In order to minimise distortion of the optical surfaceprovided by the membrane 12, and any consequential optical aberration,it is necessary to hold the membrane 12 under tension between the frontand rear rings 18, 20. In accordance with the present invention themembrane 12 is held at a surface tension of at least about 180 N/m,preferably at least 200 N/m.

Further, as mentioned above, the surface tension in the membrane 12should be stable enough over the working life of the assembly 10 andenvironmental conditions to provide a substantially constant load in thebalance of forces between the tension in the membrane 12 on the one handand the beam bending reaction force of the rings 18, 20, the pressure ofthe fluid 60, the force at the control points and/or hinge pointsmentioned above and any parasitic forces (such as from the receptacle24, or friction).

FIGS. 4 and 5 illustrate schematically a method in accordance with thepresent invention for pre-tensioning the membrane 12 to a tension of atleast 180 N/m, conditioning the membrane 12 such that it holds this loadstably for an extended period of time and assembling the assembly 10incorporating the pre-tensioned membrane 12. In some embodiments usingthe method of the invention, the membrane 12 may hold a substantiallyconstant surface tension of at least 180 N/m for a period of at least 12months.

With reference to FIG. 5A, a sheet 112 of polyether polyurethaneElastollan® 118510A as mentioned above, having a sheet thickness ofabout 380 μm is held in a circular clamp 114 to define a circular areaof the sheet within the clamp. The clamp is fixedly secured by a jig(not shown) directly beneath a selectively operable press 101 with thesheet 112 arranged horizontally. The press 101 is releasably fitted withan annular inner carrier ring 102 having a cylindrical outer surface 103that is formed with an intermediate, circumferential rib 111 (best seenin FIGS. 5A-C; omitted from FIG. 4 for clarity) and carries a first PTFEO-ring 104 at its lower extremity. The outer diameter of the firstO-ring 104 is approximately half the inner diameter of the clamp 114,although this ratio is not important; it is only necessary that thefirst O-ring should fit through the middle of the clamp 101 and protrudethrough it sufficiently for the following steps to occur. The innercarrier ring 102 also has a cylindrical inner surface 110.

With the clamp 114 and sheet 112 in position, the press 101 is operatedto move the press downwards in the direction of arrow Z in FIG. 5B (seealso FIG. 4 ) first to engage with the sheet 112 and then to stretch thesheet 112. The stretching of the sheet 112 is facilitated by the firstO-ring 104, which is suitably made from a low friction material such,for example, as PTFE, to ensure the sheet slides easily over the press101 and is tensioned uniformly. The press 101 is moved downwards againstthe sheet 112 until the sheet is strained by about 40% to a biaxialtension of about 1200 N/m at the end of the stroke of the press. Thesheet 112 becomes thinner as it stretched, reaching a thickness of about220 μm corresponding to a stress of about 6 MPa, as shown in FIG. 5C.FIG. 6A shows how the thickness (in μm) of the sheet 112 changes as itis strained to about 40%. FIGS. 6B and 6C show respectively thecorresponding changes of stress (in MPa) and load (in N/m). Theplot-lines in FIGS. 6B and 6C have a plurality of distinct legs. Leg Irepresents the change in stress/load during the tensioning of the sheetas described above.

Once the sheet 112 is stretched to its target tension, the inner carrierring 102 is engaged with an outer carrier ring 105, as shown in FIG. 5D.The outer carrier ring 105 is annular having an inner surface 106 thatis slightly greater than the outer diameter of the inner carrier ring102, so that the inner carrier ring 102 forms snug fit inside the outercarrier ring 105. The outer carrier ring 105 is held fixedly by the jigsuch that the inner carrier ring 102 is entered into the outer carrierring 105 as the press 101 is moved downwards. The inner surface 106 hasa circumferential groove 107 that accommodates a second frictionfluoroelastomer (e.g. Viton®) or Nitrile rubber O-ring 108 having aninner diameter that is slightly smaller than the outer diameter of thecircumferential rib 111 formed on the outer surface 103 of the innerring 102, so that on engaging the outer ring 105 with the inner ring 102at the end of the stroke, the second O-ring 108 bumps over the ridge 111to trap the membrane 112 between the second O-ring 108 and the innercarrier ring 102. An end stop 109 on the outer ring 105 prevents theinner and outer carrier rings 102, 105 from separating from one another.

The portion of the sheet 112 that is held by the inner and outer carrierrings 102, 105 is then severed from the remainder of the sheet as shownin FIG. 5E. The inner and outer carrier rings 102, 105, with the trimmedsheet 112 held firmly between them under tension are then transferred toan oven having a temperature of about 80° C. The sheet 112 isconditioned in the oven for about 1 hour, during which time themacromolecular structure of polyurethane material comprising the sheet112 relaxes. As shown by leg II in FIGS. 6B and 6C, during this step thestress in the sheet relaxes to about 2 MPa and the tension falls toabout 440 N/m. The temperature and duration of the thermal conditioningstep may be altered, provided that the sheet 112 is caused or allowed toundergo stress relaxation. It has been found that after this step thesheet is surprisingly able to hold a substantially constant line tensionof about 200 N/m for a period of several years. Temperatures above about90° C. should be avoided as the polyurethane material may begin todegrade.

The inner and outer rings 102, 105 are then removed from the oven andthe front and rear rings 18, 20 are glued to the front and rear surfaces14, 16 of the sheet respectively using a light curable epoxy adhesive asmentioned above. Each of the rings 18, 20 is fabricated integrally witha respective circular lead frame 118, 120 and is attached to the rest ofthe lead frame by severable tabs 122, as shown in FIG. 5G. Each of thelead frames 118, 120 has an outer diameter that is slightly smaller thanthe diameter of the Inner surface 110 of the inner carrier ring suchthat it fits snugly within the inner carrier ring 102, as shown in FIG.5F, to locate the rings 18, 20 accurately with respect to the sheet 112and each other. The lead frames 118, 120 are provided with locationfeatures 124 to assist further in positioning them accurately within theinner carrier ring 102. For convenience the epoxy adhesive applied tothe whole of the lead frames 118, 120 and then cured after beingpositioned in contact with the sheet 112. For the epoxy adhesive used, atwo-stage curing process is needed; after initiation with UV light, theadhesive is then subjected to a secondary thermal curing step in an ovenat about 40° C., for about 12 hours to develop the adhesive's fullstrength. If an alternative adhesive is employed then it should be curedaccording to the manufacturer's instructions.

In some embodiments, the rear ring 20 may be attached to an annularsupport disc (not shown) of the kind described in WO 2013/143630 forreinforcing the rings 18, 20 against the tension in the sheet 112 in theplane of the sheet. The support disc is not shown here for clarity.Typically the rings 18, 20 have protruding tabs (not shown) atpredetermined locations around the rings 18, 20 for connecting the ringsat those locations to the housing 40 at hinge points, or to the controlmechanism at actuation points, as described in WO 2013/144533, WO2013/144592 or WO 2015/044260. The tabs are also omitted from thedrawings for simplicity.

In the one embodiment, the outer face 14 is then coated with a thinlayer (not shown) of the fluorinated polymer barrier material (OF 210™,Canon Optron, Inc) to form a protective layer as described above. Thebarrier material is coated onto the outer face 14 under vacuum byphysical vapour deposition (PVD) to a thickness of about 10 nm.

A fluorinated polymer barrier layer that is coated onto the outer face14 by PVD deposition is satisfactory for use in many situations, but adisadvantage is that there are no interfacial bonds between the outerface 14 of the membrane and the polymer coating. As a result, the PVDcoating may be fragile with a risk of wearing off, for instance bytouch. In the other embodiment mentioned above, the outer face 14 of themounted pre-tensioned membrane 12 is coated with a layer ofsilica-filled cross-linkable polyurethane acrylate material instead ofthe fluorinated polymer material. The use of a barrier material that iscompatible with the membrane material allows the formation of stronginterfacial bonds between the barrier layer and the membrane 12 asresult of interactions at the molecular level. An aromatic polyurethaneacrylate material, for example, may be suitable for coating athermoplastic aromatic polyurethane membrane 12. The inclusion ofacrylate moieties within the barrier material allows the barriermaterial to be cross-linked after coating onto the membrane 12 forincreased stiffness and hardness. The inclusion of a small amount ofphotoinitiator within the material allows curing to proceed by exposureto UV light.

Suitable acrylic-modified polyurethane materials include UV1 andCeranoshield, which are commercially available from PPG Industries, Inc.Barberton Speciality Chemicals Plant, Barberton Ohio, and G-NT200 whichis available from Lens Technology International of La Mirada Calif.

The inclusion of silica nanoparticles affords added stiffness andscratch resistance. The concentration of nanoparticulate silica includedin the barrier material may be varied according to the desiredproperties of the coating, but typically the barrier material contains50-60% wt. silica. In one embodiment, the acrylic-modified polyurethanematerial may include about 52% wt. silica. If a thinner barrier layer isdesired, the silica-filled polyurethane barrier material may be dilutedwith a suitable solvent such, for example, as an acetate or alcohol,prior to application to the face of the membrane 12 as described below,which may reduce the concentration of silica particles to the range7-10% wt. which would still be sufficient to impart a degree of hardnessto the coating. Generally in accordance with the invention, thenanoparticles may have an average diameter in the range 50-200 nm,typically about 50-100 nm.

The acrylate-modified polyurethane barrier material may be applied tothe outer face 14 of the mounted membrane 12 by spin coating, butpreferably ultrasonic spray coating is used, which has been found toachieve a thickness well below about 1 μm. The use of ultrasound causesthe polyurethane barrier material to be atomised into tiny droplets,which are then sprayed over the face 14 of the membrane 12 in the formof a thin film.

Thermoplastic polyurethane of the kind used for the membrane 12 ishydrophobic in nature and has a contact angle ranging between 95-105°. Alower contact angle is generally needed to wet the surface uniformly topromote good adhesion between the face 14 of the membrane 12 and thebarrier layer. In order to attain a lower contact angle and betteradhesion, the outer face 14 of the membrane 12 is subjected to plasmatreatment (air plasma) prior to coating with the barrier material. Thisserves to activate the surface and, as a result, the contact angle isreduced to the region of 78-83°. This may be tested using dyne ink,whereby the surface energy after plasma exposure is increased from 38-40dynes/cm to about 48-52 dynes/cm.

After activating the face 14 of the membrane 12 as described above, thepre-tensioned membrane 12, still mounted between the inner and outerrings 102, 105, is transferred to a coating chamber where the coating ofsilica-fill sd acrylate-modified polyurethane is sprayed onto the face14 by ultrasound spray coating as mentioned above. After coating, thecoating liquid on the membrane 12 is cured under UV exposure usingMercury vapour H-bulb. The mercury lamp has an output in the shortwaveUV range between 220-320 nm, and a spike of energy in the long-waverange at 365 nm.

A cured, silica-filled polyurethane coating of the kind described aboveprovides a stiff, hard barrier layer on the outer face 14 of thepre-tensioned membrane 12 having elastic modulus of about 1 GPa. Thisprovides an additional advantage as described in more detail below, inthat as the tension in the membrane 12 decreases slightly during thesubsequent assembly steps, the barrier layer is compressed.

In other embodiments, the mounted membrane may be coated on its outerface additionally or instead with other coating materials known in theart such, for example, as single or multi-layer anti-reflectioncoatings.

The dish-shaped receptacle 24 is pre-assembled with the rear lens 34 bybonding the front face 32 of the lens 34 to the rear face 30 of thereceptacle 24 using a 25 μm layer of PSA as mentioned above. Thepre-assembled lens 34 and receptacle 24 are then attached to the rearring 20 as shown in FIG. 5H by bonding the peripheral lip 22 of thereceptacle 24 to the rear ring 20 with the epoxy adhesive and curing thesame. The sheet 112 is then trimmed between the rings 18, 20 and thelead frames 118, 120 as shown in FIG. 5I leaving the membrane proper 12held between the rings 18, 20. At this stage, the rings 18, 20 are stillattached to the lead frames 1 18, 120 by the tabs 122.

With reference to FIG. 5J, the front and rear retainers 48, 46 are thenassembled around the rear lens 34, dish-shaped receptacle 24 and rings18, 20 and membrane 12 to enclose the rear lens 34, dish-shapedreceptacle 24 and rings 18, 20 and membrane 12 as described above and toform said housing 40. The tabs 122 between the lead frames 118, 120 andthe rings 18, 20 are then cut to detach the assembly 10 from the jig. asshown in FIG. 5K.

Thereafter the enclosure 54 formed by the rear wall 26 of thedish-shaped receptacle 24, the membrane 12 and the rear ring 20 isfilled under vacuum with pentaphenyltrimethyltrisiloxane as the fluid 60through a fill-port (not shown) in the housing 40 and side wall 26 ofthe dish-shaped receptacle 24. As described above, alternative siliconeoils may be used instead if desired. Filling is continued until thefluid 60 contacts the whole of the inner face 16 of the membrane 12continuously as shown in FIG. 5L. Desirably the enclosure may beover-filled to a certain degree, causing the membrane 12 to distendoutwardly. Suitably the enclosure may be over-filled to a membranecurvature of about +1.0 dioptres. This serves to stabilise the loadedmembrane support structure comprising the rings 18, 20 and allows forabsorption of some of the fluid 60 by the membrane 12.

Over time the membrane 12 tends to absorb an amount of the fluid 60 fromthe enclosure via its inner face 16 which is in contact with the fluid60. In the present embodiment, the membrane 12 may absorb up to about15% of its weight of fluid. This causes the membrane 12 to swell-relax,further losing tension. Desirably this process may optionally beaccelerated in accordance with the present invention by incubating thefluid-filled assembly 10 at about 50-51° C. for about 24 hours. This isshown in leg III of FIGS. 6B and 6C, with the final tension in themembrane being about 220 N/m and the final stress being about 1 MPa,which is equivalent to a strain reduction of about 5%. During thisprocess the curvature of the membrane also decreases from about +1.0dioptres, as mentioned above, to about +0.5 dioptres. In this way, themembrane tension in the finished assembly 10 is already substantiallystabilised.

Where the membrane 12 carries a silica-filled, cross-linked polyurethanecoating having elastic modulus of about 1 GPa on its outer face 14 asdescribed above in relation to the other embodiment, the coating iscompressed when the membrane and rings sub-assembly 12, 18, 20 isreleased from the lead frames 118, 120 by cutting tabs 122, and thus theelastic forces in the coating operate in the opposite direction to theelastic forces in the membrane 12 which is tensioned.

The change in stress σ_(m) of a biaxially strained membrane of modulusE_(m) subject to a small strain ε is given by equation (I):Δσ_(m)=2E _(m)ε  (I)

As the membrane 12 relaxes during incubation and swelling, it undergoesa negative “settling” strain that reduces its tension, while putting thecoating into compression. The line tension in the membrane 12 is equalto the stress σ_(m) in the membrane multiplied by its thickness T_(m).The negative strain serves to put the membrane 12 and coating into equaland opposite line tensions:T _(m)(σ_(in)−2E _(m)ε)=2T _(c) E _(c)ε  (II)

where T_(c) and E_(c) are the thickness and modulus of the coatingrespectively.

Applying equation (II) to a membrane 12 having a thickness T_(m) ofabout 200 μm and a modulus E_(m) of about 20 MPa at an initial biaxialstress of about 1 MPa and a coating having a modulus E_(c) of about 1GPa, the settling strain would be limited to about 1% with a coatinghaving a thickness T_(c) of about 6 μm. In this way, the force appliedto the rings 18, 20 would be minimised to alleviate unwanted in-planebending of the rings 18, 20 without the need for an annular support deskof the kind described in WO 2013/143630 for example. In the otherembodiment, the coating has a thickness of about 1 μm, but even at thisthickness, the compression of the coating is enough to have asignificant effect on the mechanics of the membrane 12 such that itserves to prevent some or all of the tendency of the rings 18, 20 toin-plane collapse.

The protective layer on the outer face 14 of the membrane 12 preventsthe egress of absorbed fluid 60 from the front face of membrane. Suchegress would be undesirable as the fluid 60 might form droplets on thesurface of the membrane 12 thus impairing its optical properties.

FIG. 7 shows a scattergram of measured line tensions over time forsixty-six individual polyurethane membranes that have been tensioned andthermally conditioned in accordance with the invention and held incontinuous contact with a body of silicone oil. As can be seen, themembranes hold the tension substantially constant for an extended periodof more than two years (FIG. 7 shows up to 796 days). It is likely thatthe membranes are able to hold the tension substantially constant foreven longer, but that has not yet been measured.

In yet another embodiment, the inner face 16 of the membrane 12 may becoated with a barrier layer (not shown) of a suitable hydrophobiccoating material of the kind described above for use on the outer face14. In this way, the ingress of fluid 60 into the membrane 12 may beprevented or at least retarded. In such a case, the manufacturingprocess would not need to accommodate swell-relaxing of the membrane 12owing to the absorption of fluid 60 avoiding the need to incubate thefilled assembly at an elevated temperature to accelerate swell-relaxingof the membrane and it may be possible to biaxially tension the membrane12 to a slightly lower initial tension.

The effective modulus E_(ef). of a membrane having a modulus E_(m)carrying a coating having an elastic modulus E_(c) is given by theequation:

$\begin{matrix}{E_{eff} = \frac{{E_{m}T_{m}} + {E_{c}T_{c}}}{T_{m} + T_{c}}} & ({III})\end{matrix}$

The thickness T_(c) of the coating may be measured optically, while thecombined thickness T_(m)+T_(C) of the membrane and coating may bemeasured using a thickness gauge.

The modulus of the membrane when coated and uncoated may be measured byholding the membrane around its edge in steel rings clamped to a sealedvessel that is pressurised to a pressure P. As a result of pressure inthe vessel, the membrane bulges outwardly, and the maximum outwarddisplacement h of the membrane can be measured using a laser heightmeasuring system. From this, the tension, biaxial stress and strain ondeforming the membrane from flat to nearly spherical, and hence theeffective modulus of the coating and membrane, or of the membrane only,can be calculated.

The invention claimed is:
 1. An adjustable fluid-filled lens assemblycomprising an enclosure, one wall of which is formed by a thermallyconditioned and bi-axially tensioned thermoplastic polyurethane membranehaving a thickness in the range 100 to 300 μm and an elastic modulus inthe range 10 to 200 MPa which is mounted to a peripheral supportstructure, and which is filled with a fluid; wherein the membrane issaturated with said fluid, is coated on its outer face with a barrierlayer to said fluid, and the membrane holds a substantially constantsurface tension of at least 180 N/m.
 2. The adjustable fluid-filled lensassembly of claim 1, wherein the membrane absorbs up to about 20% fluidby weight of the membrane.
 3. The adjustable fluid-filled lens assemblyof claim 1, wherein the membrane holds a substantially constant surfacetension of at least 180 N/m for a period of at least 12 months.
 4. Theadjustable fluid-filled lens assembly of claim 1, wherein the membraneis non-circular.
 5. The adjustable fluid-filled lens assembly of claim1, wherein the enclosure is compressible.
 6. The adjustable fluid-filledlens assembly of claim 5, wherein the peripheral support structure forthe membrane comprises one or more bendable rings that are arranged tohold the membrane around its edge.
 7. A pair of spectacles comprising atleast one adjustable fluid-filled lens assembly of claim
 1. 8. Anadjustable fluid-filled lens assembly comprising an enclosure, one wallof which is formed by a thermally conditioned and bi-axially tensionedthermoplastic polyurethane membrane having a thickness in the range 100to 300 μm and an elastic modulus in the range 10 to 200 MPa which ismounted to a peripheral support structure, and which is filled with afluid; wherein the membrane is coated on its inner face with a barrierlayer to said fluid, and the membrane holds a substantially constantsurface tension of at least 180 N/m.
 9. The adjustable fluid-filled lensassembly of claim 8, wherein the membrane is free of said fluid.
 10. Theadjustable fluid-filled lens assembly of claim 8, wherein the membraneholds a substantially constant surface tension of at least 180 N/m for aperiod of at least 12 months.
 11. The adjustable fluid-filled lensassembly of claim 8, wherein the membrane is non-circular.
 12. Theadjustable fluid-filled lens assembly of claim 8, wherein the enclosureis compressible.
 13. The adjustable fluid-filled lens assembly of claim12, wherein the peripheral support structure for the membrane comprisesone or more bendable rings that are arranged to hold the membrane aroundits edge.
 14. A pair of spectacles comprising at least one adjustablefluid-filled lens assembly of claim 8.